1. Field of the Invention
The present invention generally relates to a thermal-insulation container with electric heater, and more particularly relates to a thermal-insulation container with electric heater having improved thermal efficiency.
2. Description of the Related Art
A thermal-insulation container with electric heater having an inner container made of metal, an outer container and a thermal barrier interposed therebetween with an electric heater provided in the middle and bottom portions of the inner container is well known in the art.
In such a thermal-insulation container with electric heater of the prior art, since the inner container is made of a metal having a thermal conductivity, heat is transmitted therethrough to be dissipated from a container opening, disadvantageously resulting in increased power consumption. Such a heat loss becomes remarkable especially when the container has a large opening. Generally, heaters used for such thermal insulation containers of the prior art include sheathed heaters, mica-insulated heaters, and the like.
In those thermal-insulation containers with electric heater is provided with an electric heater on the lower external surface of the inner container, it is necessary to additionally provide a screw receptacle for ensuring a secure contact of the heater to the inner container at its lower part. Further, in such a type of thermal-insulation container with electric heater of the prior art having an air pump to pump up the hot water from the container, it is necessary to provide a liquid port in the bottom of the inner container. Therefore, as the bottom of inner containers becomes more complicated in shape, it has become necessary to manufacture heaters therefor more elaborately so as to fit well into a place in the bottom they are mounted.
Further, of the heat generated by a heater as described above, specifically the heat that is generated from the surface contacting the inner container is transmitted thereto by conduction to be used efficiently for heating and keeping warm water or other liquid in the inner container.
However, the heat generated from the closed space side of the heater escapes as radiant heat towards the outer-container side and does not contribute to heating and thermal-insulation efficiency, thus tending to increase heat loss. Thus, these thermal-insulation containers of the prior art disadvantageously involve a lower thermal efficiency. In order to make up for this, it has been proposed to provide a protruding heater retainers in the bottom of the inner container to increase the contact area between the heater and the bottom, or alternatively to provide a multitude of semispherical protrusions in the bottom of the inner container to increase its surface area.
Accordingly, it is an object of the present invention to provide an improved thermal-insulation container with electric heater having a good thermal efficiency, and also which is easy to manufacture.
In one aspect of the present invention, a thermal-insulation container with electric heater having an inner container and an outer container, the inner container being housed in the outer container integrally therewith so as to define a closed space separating the same from the outer container, comprises a highly-resistive metal oxide film formed on an external surface of said inner container facing said closed space, and an electrode unit provided on an external surface of the highly-resistive metal oxide film facing the closed space for applying an electric current to the highly-resistive metal oxide film, wherein the highly-resistive metal oxide film reflects incident infrared rays by at least 40%.
In another aspect of the present invention, a thermal-insulation container with electric heater having an inner container and an outer container, the inner container being housed in the outer container integrally therewith so as to define a closed space separating the same from the outer container, comprises a highly-resistive metal oxide film formed on an external surface of said inner container facing said closed space, an electrode unit provided on an external surface of said highly-resistive metal oxide film facing said closed space for applying an electric current to said highly-resistive metal oxide film, and a highly-reflective metal oxide film reflecting incident infrared rays by at least 40% provided on an external surface of said highly-resistive metal oxide film facing said closed space through the intermediary of an insulator film interposed therebetween.
In yet another aspect of the present invention, it is preferred that the outer container and the inner container are made of glass and that the highly-resistive metal oxide film, the insulator film and the highly-reflective metal oxide film are transparent.
In still another aspect of the present invention, it is preferred that the inner container is joined with the outer container only at and along a peripheral edge of an opening of the thermal-insulation container with electric heater.
According to the present invention, a thermal-insulation container with electric heater has a double-walled structure in which an inner container is housed in an outer container integrally therewith so as to define a closed space separating the same from the outer container, and on an outer surface of the inner container facing the closed space is formed a metal oxide film having a high resistivity (herein referred to as a highly-resistive metal oxide file) and on the highly resistive metal oxide film facing the closed space is provided an electrode unit for applying an electric current to said highly-resistive metal oxide film.
When passing an electric current through the highly-resistive metal oxide film, heat is generated by its electric resistance and thus the highly-resistive metal oxide film serves as a heater. In order to make the heat generated by the highly-resistive metal oxide film act efficiently to heat the inner container, a metal oxide film having a high reflectance (herein referred to as a highly-reflective metal oxide film) may be provided on the surface of the highly-resistive metal oxide film facing the closed space. However, if this highly-resistive metal oxide film itself is formed as a coating having a high reflectance (low emissivity), even a single layer of such highly-resistive metal oxide film will bring forth a sufficient heat insulation efficiency to substantially reduce radiant heat loss from the inner container.
In a preferred embodiment of the thermal-insulation container with electric heater according to the present invention, a highly-reflective metal oxide film is provided on the surface of the highly-resistive metal oxide film facing the closed space through the intermediary of an insulator film interposed therebetween.
Since such a highly-reflective metal oxide film has a low emissivity (that is, a high reflectance), it can reduce the quantity of heat emitted to the closed space side (heat loss) as radiant heat of the highly-resistive metal oxide film.
The insulator film electrically insulates the highly-resistive metal oxide film and the highly-reflective metal oxide film, so that the current flows only through the highly-resistive metal oxide film. The insulator film also acts to prevent sodium (Na+) ions or other impurity ions contained in the highly-resistive metal oxide film from diffusing and migrate into the highly-reflective metal oxide film when the latter film is formed, consequently preventing a reduction of the reflectance efficiency of the highly-reflective metal oxide film and its effectiveness to prevent the radiant heat transmission. In other words, the insulator film prevents a migration of impurity ions from the highly-resistive metal oxide film into the high reflectance oxide film and thus plays an important role in the prevention of performance degradation of the highly-reflective metal oxide film.
Further, according to the present invention, it is preferred that the closed space separating the inner container from the outer container is hermetically sealed, and the closed space may be evacuated or filled with an inert gas such as krypton, xenon or argon gas which is thermally less conductive than air so that it can serve effectively as a thermal barrier to improve its thermal-insulation efficiency. Especially, if the closed space is evacuated, it can maintain its high reflectance (low emissivity) over a long period of time, because under vacuum the highly-reflective metal oxide film will neither adsorb moisture etc. nor be coordinated with excessive oxygen and it is not oxidized even when functioning as a heater to be placed under a high temperature condition.
In a preferred embodiment of the present invention, the thermal-insulation container with electric heater in which its inner and an outer containers are made of glass and both the highly-resistive metal oxide film and highly-reflective metal oxide film are transparent as well as the insulator film, and therefore the content of the inner container can be readily visualized.
Although the prior art heat-insulating containers made of glass exhibit a substantial heat loss due to heat conduction through a pad used for the positioning of inner container and outer containers, the thermal-insulation container with electric heater according to the present invention achieves a further improvement in thermal-insulation efficiency, because its inner and outer containers are joined only at its opening so that the closed space defined between the inner and outer containers serves as a heat barrier to suppress heat loss attributable to heat conduction.